Alarm Circuits-circards 13

8/6/2019 Alarm Circuits-circards 13

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Wireless World Circard Series 13: Alarm CiFlame, smoke and gas detectors

Component valuesTr,: TIS34

Tr,: BC126Tr,: BC125Tr,: BC125Tr,: BC126D,: lN914D,: lN4002C,: 1nFR1, R,: 1 5 MR,: 2 . 2 k _R,: 1 2 k

R,: 6 . 8 k R,: l . 5 k R7 R9: 1 2 kR8: 820RR10: 1 5 kRll: 2 . 7 k Rl2:4 . 7 k Rl3: 2 2 kSemiconductorcritical but TISneed selection bparameter spreaea bYW

Flame detectorA flame offers a low conductance path to ground. In serieswith R1,R2, that conductance defines a range of potentials onthe gate of Tr,, that leaves the emitter of Tr, at a high enoughpotential to keep D1 out of conduction, but not so high as tobring Tr, into conduction via R,. Hence Tr,, Tr5 conductholding on the relay-interlocked with the supply for fail-safeoperation. If the flame is extinguished Trl gate goes high,driving Tr 4 on via Trz,R7. This removes the drive from Tr,,

Tr5 and the relay. A short circuit to ground at the inputreduces the base potential of Tr, bringing D1 into conductionand cutting of Tr3 and hence the output.

is high requiring a high input resistance buffer; tconventional.

Smoke detector

The mid-section of the circuit offers a window action withthe relay being held on for a restricted range of flame resist-ances, higher and lower values giving drop-out. The resistance

When detecting the interruption of light byavoid the effects of ambient illumination etc., themay be chopped at source and the resulting a.

(see over) used via buffer Tr, to trigger the circuit around Tr3, Tr,. This prevents the potentiaR10, from rising sufficiently to fire the thyristor. Ia horn having an interrupter switch in series withthyristor can cease conduction on removal of th(alternatively a.c. drive to the load would be req


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sensor buffer ;

Tr1: LS400Tr2-4: 2N712Tr5: C106FR1: 1 kR2, R8:1 O O kR3: 1 5 kR4: 4 7 O kR5-7, R9: 1 O k

monostable i

R10, 3 . 9 k C1, 16FC2, C4: 22nFC3: 0.lFC5: 50FC6: 4.7nFTransistor types notcritical.

Tr,: BC126Tr2: TIS43C1: 0.22FLS: 8 to 80 R

R1: 470KR2: 3 . 3 k R3, R5: l O kR4: I O O kR6 , R7: 1 k

brings Tr, into conduction, C1 charges until the uTr2 fires and the cycle recommences. The audible nloudspeaker rises from a succession of clicks to a ctone as the gas concentration increases. A Schmwould allow relay drive, while the audible alarm

transferred to the flame-detector circuit, for examp

Gas detectorFurther reading

A particular gas-sensor (TGS from Figaro Engineering,Transducer detects gas, Electronic Components, 6

Shannon, Ireland) has two fine wires embedded in a semi-p.18.

conductor. One is used to heat the material, with the resist-Wolfram, R., Fail-safe flame sensor provides control

ance between it and the second being reduced on the absorptionElectronics, 31 Aug. 1970, p.68.

of deoxidizing gas or smoke. The sensor is sensitive toMarkus, J. (ed.), Smoke detector receiver, in E

concentrations of


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Wireless World Circard Series 13: Alarm CirBridge circuits

ComponentsICs: 741, Vs15vR1 to R4: I O k , R5: 1 MBridge voltage: 1.5V (Fig. 2)

Circuit descriptionThree bridge configurations are shown. In each case the bridgeis composed of four resistors, R1 to R4, and the circuits are

basically Wheatstone bridges with balance occurring forR1/R2=R3/R4. Substitution of impedances Z1 to Z4 wouldleave the balance requirements unchanged, and other variantssuch as the Wien bridge can be produced. For resistiveelements it may be possible to supply the bridge and amplifierfrom a common d.c. supply and a high-gain op-amp detectsdeparture from balance. A small amount of positive feedbackvia R5 helps reduce jitter in the output when close to balance,but gives hysteresis to the balance sensing.* If a separate supply is required for the bridge, one bridgebalance point may be grounded, removing the need for highcommon-mode rejection for the amplifier. The errors in allthese circuits include voltage offset of the amplifier, l-5mVfor untrimmed general-purpose op-amps, and input currents/offset, 1OnA to 1A for conditions as before. For balancedetection to within 0.1% this implies bridge voltages in excess

of1V and currents of up to 1mA.

0 By opening the bridge and embedding the amnetwork as shown, balance is achieved for the samship between the resistances, but with input and with respect to ground. This circuit has an outplinear function of the departure of R2 from condition (R1,R3,R4 assumed constant as referencFor d.c. applications the input may be one or osupply voltages. In all cases best sensitivity is aR1/R+l. If the resistor whose value is being sehave a low resistance, power wastage is avoided the other pairs of resistances high.l Another method of achieving input and outputreferred signals, is to use an amplifier with push-and single-ended input. A simple case is the singas shown where the power supply, if properlycloses the bridge when used for a.c. measurem


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-l-_. I I___-_The example shown would pass all frequencies except the voltage via a resistor chain with very good stabinotch frequency defined by l/RC, though with appreciable ratio of their values; the absolute values are notattenuation near the notch. for such an application. The lower-threshold0 For many purposes, the availability of a centre-tapped (trigger) when held high prevents any output chasupply provides a phantom-bridge action. If the ratio of 1 is assumed high) regardless of the status of the respositive to negative supplies remains constant then taking The reset terminal regains control only when the trione input of the sense amplifier to the centre-tap leaves only falls below the level accurately defined by thea half-bridge externally. Used for example with photodiodes, divider. With the trigger taken from an externathe output voltage is proportional to the unbalance currents divider containing the required sensing element thin the diodes i.e. to the degree of unbalance in the illumination balance sensing can be obtained.of the diodes. Because the diodes act as constant-currentdevices the circuit is much more tolerant of drift in the Further readingcentre-tap than for purely resistive elements. The negative Markus, J. (ed.), Bridge circuits, in Electronicfeedback gives a linear output-unbalance characteristic. Manual, McGraw-Hill, 1971, pp.84-9.Reversal of the amplifier input terminals would give positive Graeme, Tobey & Huelsman, Operational Afeedback, introducing a switching action and hysteresis as McGraw-Hill, 1971.

in the first diagram.l Some i.cs have internal potential dividers which can Cross referenceseffectively form part of a bridge. The 555 timer, for example, Series 1, cards 9 & 10, series 9, cards 1 & 11.has its two comparators tapped at + and 8 of the supply Series 13, cards 1 & 3.

1974 IPC Business Press Ltd.


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Wireless World Circard SeriesIs: Alarm CircTime delay and generator circuits

Circuit descriptionAn i.c. such as the 555, with internal comparators driving aset-reset flip-flop offers great flexibility in the design of alarmsystems. With pin 2 high, the capacitor is held low via pin 7.A negative-going edge on 2 allows R1 to charge Cz until thepotential on 6 passes 2 VJ3, when the original state is restored.0 Linking the inputs of the two comparators (2 and 6) to thedischarge path (7) causes the potential at the common point

to cycle between VS/3 and 2 VS/3, set by an internal potential-divider. For both circuits the output has switching character-istics comparable to a t.t.1. gate because of a similar totem-poleoutput stage. An audible alarm is available by connecting aloudspeaker (3-2552) between Vs and pin 3. If V, is +5V,the on/off condition of the alarm may be controlled by

driving pin 4 from the output of a t.t.1. gate.0 An astable can also be constructed by feedbacoutput to the paralleled comparator inputs. Whenis high, C1 is charged positively through R1 untilthreshold is passed; the output switches low andcharged until the trigger value set by pin 2 is passis set by the less well-defined output amplitudfrequency is less stable than the basic circuit. Addvaries mark-space ratio.0 If the reset terminal 4 is coupled to an RC nshown, then a time-delay can be introduced at before which firing of the circuit as a monostabachieved.


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l A monostable using c.m.o.s. inverters can use very high-value resistors, giving time delays of >Is with capacitors of


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Wireless World Circard Series 13: Alarm Cir

Level sensing and load driving

Circuit descriptionThe basic level-sensing circuits shown may be used with orwithout positive feedback, to obtain an output change as theinput passes a defined level or levels. For R*-+~o, RI-d,amplifier gain determines the range of input voltages for whichthe output is not switched hard to one or other extreme.(Typically 1 to 20mV for comparators, required to operate athigh speeds; 0.1 to 5mV for op-amps where accuracy oflevel-sensing makes their slower operation an acceptable penalty.)Hysteresis introduced by positive feedback allows the circuitto latch into a final state after the first excursion through agiven level, provided the input cannot reverse its sensesufficiently to pass back through the other switching level.These circuits can thus perform the combined functions oflevel-sensing and set-reset action required in many alarms if forexample the signal is a positive-going voltage initiating theset action, while the reset action is a negative-going pulseover-riding the former e.g. a resistor taken from the non-inverting input to the negative rail.

Tr,: BFR41, Tr,: BFRSlVsf6V, RL: 20052R1 to R,: 10052, R,: 1.2kQ R: 1kSLIC: 7110 An adaptation of the output stage shown in Fioutput when the p.d. across either R1 or Rz ex0.6V. In the former case this corresponds to a povoltage defining sufficient positive supply curi.e. VI~RJR,-0.6V. Similarlyanegativeinput voltathe output via Trl. The switching action is not sharp as it uses only the gains of the transistors.0 A standard window comparator gives sharpbut requires two amplifiers/comparators and stilladditional transistor is an output swing comparabvoltage is required e.g. for efficient switching of letc., particularly at higher currents.


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Wireless World Circard Series 1 3 : Alarm Cir

Applications of 555 timer

Circuit descriptionThe 555, designed as a timing circuit with either monostableor astable operation, has internal circuit functions that allowit to be used for many other purposes. In alarm systems, thepower output stage that permits currents of either polarityof up to 200mA (though 50mA minimizes voltage losses)means that lamps and relays can be driven quite readily.When used as an astable circuit the output square wave canbe applied to a loudspeaker to give an audible alarm, while avoltage fed to the control terminal modulates the frequencyfor warble or two-tone effects. As a monostable circuit it canbe used to provide delays from microseconds to minutes,allowing, for example, a warning alarm to be held for a

defined period of time after the appearance of the conditionbeing detected. In such cases the condition (closure of aswitch in a burglar alarm for example) is converted into anegative-going pulse, applied to the trigger input. A furtherapplication for the device involves the controlled hysteresis

provided by the two comparators biased from potential divider. With VI>V,,f, the output is drivvia the flip-flop which ignores any further exV, about Vrerl in either sense. When Va falls belflip-flop is reset, the output going positive. In circuit VI= V,,V,,r,=2Vs/3, V,,r,=Vs/3 and this charged and discharged between Vs/3 and 2Vs/Typical performanceIC: NE555V (Signetics), Vs + 1OVR,: 2.2kQ, R,: 1OkDk=0.6, D,: 5.6-V Zener diodeUpper set point: 5.7V ( VZ)Lower set point: 4.75V (Vz/2k)Output swing: 9V for RL > 25052IfR1, Dz omitted, VM,= 2 V, Vrer,= Vand set po2V and V/k.


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Component changesIC: Motorola MC1455 Separate comparators could beused with independent reference voltages or a singlecomparator with hysteresis defined by feedback-see Series 2.4.5 to 18V. At low voltages the saturation voltagesat the output may not allow adequate drive to

electromechanical/filament lamp loads.Any network to provide constant voltage at controlinput. Voltage may be to within 1V of common lineor positive supply, but for optimum performanceshould be close to 2Vs/3.lk to 1MQ. At low values, excessive loading ofsource; at high values inaccuracies due to thresholdcurrent of up to 0.25,uA.

Circuit modifications

l Use as battery charger illustrates method well (above).Upper threshold when ~,VL=V,; lower threshold whenk,VL=Vz/2. When upper threshold is exceeded output at

pin 3 reverse-biases diode Dz and battery dischargewhen present. As voltage VL falls below lower voltage at pin 3 rises and charges battery througresistor Rz. Hysteresis may be reduced towardsVzlk,+ V&k,.l To increase hysteresis, the potential at pin

reduced following a transition through the upperThis may be done as in Fig. 2 by using output pdiode-both thresholds are varied if the diode isby a resistor.l The increased swing similifies the triggering of 555 used as a Schmitt trigger, as the capacitor Fig. 2 can approach zero. Complete alarm systebased on such circuits combining level sensing, tiand waveform generation, as well as audible alarm

Further readingFour articles, by De Kold, McGowan, Harvey Electronics, 21 June 1973, pp.128-32.

Vs:

RI, D,:

R,:

0 1974 IPC Business Press Ltd.


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Wireless World Circard Series 1 3 : Alarm CircFrequency sensing alarm

at a lower frequency (f/20), and hold on during sior for temporary interruptions of the signal.

The upper frequency in the band mode or the d

datum mode is set by t1 and the lower band-fretl+ tr. The circuit provides frequency-sensing functo comparators Schmitt-triggers and window-com

Circuit descriptionThe circuit is a monolithic m.o.s. i.c. which uses external

RC elements to fix the frequencies at which the circuitorovides a switching action. If does so via two senarateswitching times defin;d by CIR, and CaRa, as from a pair ofmonostable circuits with the second time interval beinginitiated at the end of the first. The input may be a repetitivesignal of arbitrary waveform, provided the amplitude is inexcess of 1OOmV pk-pk (though it should not exceed 20Vpk-pk). Internally this is presumably squared by a Schmitttype of circuit to trigger the monostables. Three distinctconditions may exist; if the period of the received signal is

t=l/ and the two delays are t,=k/C,R, t,=kjC,R,, thent f i .open, band mode, on for fi>f >fi.link to pin 5, output toggles at f/20 when in


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(a) -=EI 0R

(b)-4

Component changesVs: -12 to -22V some samples operate with reducedaccuracy down to -8V.Vin: 0.1 to 20V pk-pk

freq. set points: O.OlHz to SOkHz.response time : within 5 to 10 cycles of receipt of correct

frequency.1OOk to 1MB250pF to 1pF1OnF to 1pF (not critical)

0Y 8tkcIl Variation in both frequencies while retaining aconstant ratio of f i : f i (the equivalent of a constabe achieved by varying the common bias applresistors. If strong dependence on supply voltagavoided the bias voltage should be supply-propin (b).l Constant-current sources allow linear controagainst a separate reference voltage, which may

proportional.l Filament lamps may be driven via an addisistor, currents up to lOOmA or so being providedon right. Direct drive of reed relays, l.e.ds is possicurrent is marginal.

Circuit modifications Further readingl As the lower frequency in the band mode is affected bytime constant CaRa in the original circuit while the upper Volk, A. M. Two i.c. digital filter varies passbfrequency is not, variation of R, increases the band by Electronics, 15 Feb. 1973, p.106.variation of its lower bound only. For C,= C,= C, variation McKinley, R. J., Versatile digital circuit filters hin the tapping point of RC in (a) at left leaves the sum of the

or bands. Electronics, 21 June 1971, p.66.

time constants unchanged at (&+Rn+Rc)C i.e. it is the FXlOl : Consumer Microcircuits data sheet D/026lower frequency that remains constant while the upper Cross referencesfrequency is charged. Series 1, cards 6 & 7.



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Digital alarm annunciatorsICs: 1-3, 5, 7-15

)xSN74004,6, #x SN7410

r:3.3k52, R,: 68Q

Circuit functionIt is assumed that a fault condition is the opening of relaycontact RLr, though any other sensor that maintains theNAND-gate input terminal at a low (0) level is adequate.A fault will turn off a safe green light and illuminate adanger red light, and operate an audible alarm. When therecognise push-button is depressed, the red light stays on,but the alarm is silenced. When the fault clears, the alarm isrestarted, the green light comes on and the red light goes off.

The recognise button is again pushed to reset cnormal state.

Circuit operationConsider the circuit in its normal state where inpuF are at zero volts (or binary zero) i.e. R=T=makes X=0(x= l), Y=O (P= 1) and hence LED1(green) and LED2 (red) is off.

If a fault occurs, RLt opens, F goes high (or i.e. F=l, causing X=0 (;iz=l), but the state ofremains as before. Hence A=l, and triggers audPushing the recognise button causes R= 1, anT=O, then Y=l@=O>, but X does not charemains on, but A=O, and alarm stops. This stmaintained until the fault is cleared.

When the fault is cleared, R=F=T=O, Y does but X=0 (Y=z=l, X=p=O). Hence LEDz is A= 1, and the alarm operates,

Final recognition of the fault clearance is obR=l, which will return circuit to its normal sR=l, F=T=O, Y=O and X=0.Depression of the test button will check LE

alarm, when started from normal state with LED

Circuit modificationX

As X, R, Y, p are available, the ex- Felusive-OR function of A can be ob- 2tained as shown. Y


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1 ; +v i +V,(+SV)

Circuit descriptionComplementary-m.o.s. devices may be. used in the circuitabove to minimize stand-by power consumption.

Normal safe condition obtains with L=A=O,F= 1. Whenthe fault-switch closes, F-t1 and since L is already high,X+0. Hence L=O, E= 1, opening gate ICr,. Also since F=O,Y+O, and hence A is forced to zero, therefore A= 1. Thistransition may be used to switch an audible alarm. Simul-taneously the oscillator gate is opened which will cause lampflashing at a rate determined by the astable frequency.

If the fault is rectified, the alarm condition is maintainedOutputs Qa,Qs, QB are set to zero when the rese

until the clear button is pressed causing C to be low. Hencedepressed. The 0 output of each flip-flop is appother two NAND gates, but not to the one assoL-+1, and will latch in this condition via memory circuit IC2

and ICI. Also E=O, thus x=0, this condition being main- itself. Hence two of the three inputs of each gateIfS, closes, for example, IC2 output goes lowtained via ICI and IC5, and the alarm is silenced. negative-going edge being applied to IC, preset teCircuit description Q&=1 (and hence q=O). Therefore ICI and ICArrangement right allows detection of first-fault occurrence inhibited and cannot respond to a fault conditionfrom three sensors Sr, Se, Ss, this number being restricted by Further readingthe number of inputs available per NAND-gate. Zissos, D., Logic design algorithms, Oxford 1972.



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Filament lamps and relays

Filament lamps are widely used as visual alarm indicatorsand often connected in the collector-emitter circuit of abipolar transistor that is switched on and saturated underalarm conditions. These lamps have a positive temperaturecoefficient of resistance with a large difference of resistancebetween the cold and hot states-see Graph 1 which is typicalfor a 6-V, lO@mA panel lamp. When switched on across avoltage source, a large current surge flows in the lamp, andswitching transistor, which then decays exponentially to itsnormal or rated value in the hot state. This surge may be

ten times the rated current, or even higher, shortens the lifeof the lamp, may destroy the switching transistor or blow thepower supply fuse. Graph 2 shows the typical initial surgecurrent characteristic of a 6-V, 60-mA panel lamp having athermal time constant of about 2ms.

When lamps are used as flashing alarms, the current is as shown in Graph 2 but the surge

successive pulses depends on the thermal time cthe time between flashes. Graph 3 shows the typicin surge current when a 6-V, 60-mA panel lamp on for 5 s then off for t ,,rr seconds.If the p.d. applied to the lamp is gradually incurrent rises in a controlled manner to its normavalue, prolonging the life of the lamp and rprobability of transistor damage. A simple arrashown above where Tr, is normally held on anwith a low value of Vc~(sat) holding Tr, and thUnder alarm conditions, the base drive to Tr, and the capacitor charges through Rn. The baseTrl rises exponentially so that the lamp surgeavoided.


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To prevent damage to Trr should the lamp become short-circuited, a resistor Rc could be included in Tr,s free collector,but this would reduce the lamp voltage in normal operation.Circuit above shows a modification that allows an almostnormal lamp voltage and also limits the short-circuit currentto the desired value by using only a small Rc value and asaturating transistor Tr,.

Relays are used to actuate alarm devices that need to be

isolated from their control circuitry for various reasons suchas their current, voltage or power requirements being incom-patible with the electronic circuitry. Circuit right, acommonly-used relay drive circuit which takes into account both theresistive and inductive properties of the relay coil. Whenactuated, the steady-state coil current is fixed by the coilresistance and supply voltage, but when Tr, is turned off theinductance of the coil causes the collector voltage to risetowards a level greatly exceeding V CC if the protective diodeD, were omitted. Diode D1 allows VCE to rise only slightlyabove Vce before the diode conducts to dissipate the energystored in the relay coil. When Tr, turns on D1 is reverse-biased and does not affect the operation. The diode must beable to withstand a reverse voltage slightly greater than VCC

0 1974 IPC

and be able to conduct the relay-coil discharge cubrief time. Transistor Tr, must have a VCE ratingVcc and be capable of carrying the relay operating

If a relay is required to operate when an input leva certain predetermined value, it may be included itrigger circuit; e.g. the relay coil and protective dreplace R1 in the basic circuit of series 2, card 2.

If the alarm indication uses a 1.e.d. or alpha-numof l.e.ds consult series 9, cards 2, 5 & 6.Further readingShea, R. F. (Ed), Amplifier Handbook, section 3 106Hill 1966.Egan, F. (Ed), 400 ideas for design, vol. 2, pp.18/1971.Cleary, J. F. (Ed), Transistor Manual, pp.202Electric Co. of New York, 1964.Industrial Circuit Handbook, section 2, SGS-Fairc

Cross referencesSeries 2, card 2.Series 9, cards 2, 5 & 6.Series 13, cards 4 & 7.

Press Ltd.


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Wireless World Circard Series 1 3 : Alarm CircSignal domain conversion

Fig. 1 Fig. 2 Fig. 3AVoltage-to-current conversionIt is often required to supply signals to relatively long trans-

mission lines in which case the signal is more convenient incurrent form rather than as a voltage. Thus, voltage-to-current converters are useful and may be realized usingoperational amplifiers especially if the load is floating.Figs. 1 & 2 show the more common forms the former beingan inverting type and the latter non-inverting. In both Figs.i= Vin/R and is independent of the load impedance, but thesource and operational amplifier must be able to supply thisload current in Fig. 1, whereas little source current is neededin Fig. 2 due to the high input impedance of the amplifier.

Fig. 3 shows another floating-load V-to-I converter whichrequires little source current ifR1 is large and allows iL to bescaled with R,, the operational amplifier supplying the wholeof the load current; iL=Vin(l/R, + RP/R1R3). The circuitof Fig. 4 is suitable for V-to-I conversion when the load is

Fig. 4

R2

grounded. When R1R3= R,R, the load current isR, and the current source impedance seen by thehigh.

Current-to-voltage conversionIf a device is best operated when fed from a volbut the available signal is in the form of a current,to-voltage converter will be required, one example bin Fig. 5. Current is fed to the summing junctoperational amplifier which is a virtual earth so tsource sees an almost-zero load impedance. Inp

flows through R1 producing an output voltage ofVvolts/amp. The only conversion error is due tocurrent of the operational amplifier which is alsummed with iin. The output impedance is very the use of almost 100% feedback.


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Wi l W ld C


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Wireless World Circard Series 13: Alarm CircPressure, temperature and moisture-sensitive alarms

Pressure-sensitive alarm

A pressure-sensitive alarm may be made using a specially-modified transistor known as the Pitran. It is a planar n-p-ntransistor having a diaphragm mounted in the top of its metalcan which is mechanically coupled to its base-emitter junction.When a pressure is applied to the diaphragm a reversiblecharge is produced in the transistor characteristics. Themechanical pressure input can be used to directly modulatethe electrical output of the transistor which may be fed tothe alarm circuitry e.g. via a comparator or Schmitt triggerwhich switches state when the input pressure to the Pitraneither exceeds or falls below some critical level. The Pitranmay be connected as a single-ended-input single-ended-outputstage, as shown left or as a differential-input balanced-output stage, as shown middle. Conventional transistor circuitdesign techniques may be used for the Pitran stages. Linear

output voltages of up to one-fifth of the total supare obtainable.

Temperature-sensitive alarmCircuit above shows the input circuitry of an alarmbe operated by the output signal from the amplifier when the temperature monitored bytransistor exceeds a pre-determined value. The tsensing transistor is a low-cost n-p-n type that ca resolution of less than 1 deg C in a temperatu

100 deg C. If the operating current of the probe made proportional to temperature, the non-linebase-emitter voltage may be minimized, being lesin the temperature range -55 to +125C. Zenethe input voltage to 1.2V and this is applied throu


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the operating current of the probe transistor. Resistor R4may be adjusted to make amplifiers output zero at 0C andR& is used to calibrate the output voltage to lOOmV/deg C,or any other scaling factor, independently of the Vout= 0condi t ion. RI,Rs 12ksE;Ra3kQ; R,5kSZ;Rs,RB 1OOkQ;D1, Da LM113; TrI2N2222; Al LM112; V hl5V.Moisture-sensitive alarmA low-cost audible alarm which operates when the electrodesof the input sensor become damp due to increase in humidity,direct contact with water, rain or snow is shown above.The sensor is conveniently made from parallel-strip printedcircuit board or commercial equivalent, so that increase inmoisture at the strips produces a very small current to Trlbase via RI which forms a high-gain compound pair with Tr,which switches hard on. Transistors Trs and Tr, form the

audible alarm multivibrator, that acts as a loacompound pair, having a repetition rate determinC,R, time constant. A piercing note at aboutproduced with RI,R,lOOkS2; RJ lkS2; C1 1OnF; TZTX300; Trs OC71; LS 8-Q loudspeaker; V +9V.

A flashing display with a rate of about 2Hz may by replacing the loudspeaker with a 6-V, 60-mA pand changing the values ofRz to 470kQ and C1 toFurther readingTingay, E. The Pitran-a new concept in pressurment, International Marketing News, p.8, 1970.Linear Applications-application notes AN3 1, AN72, National Semiconductor, 1973.

Brown, F. Rain warning alarm, Everyday Epp.208-11, 1972.


Wi l W ld Ci d


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Wireless World Circard Series 13:

Security, water level and automobile alarms

I +

Vl Y-I --Circuit descriptionComponent R, is the resistance in the search loop which ifobtained using two lOOkS2 resistors allows one to includeswitches either in series with the loop or in parallel with eitherresistor, or both. In the latter case changing a switch conditionfrom open to closed in the parallel case and from closed toopen in the series case can give rise to either a positive voltageor a negative voltage being applied to the diode bridge; thebridge is, of course, balanced initially. The diode bridgebeing a full wave rectifier will apply a negative Vba to thefollowing circuit in either case.

The bridge resistors are large valued to minimize current

drain from the battery but requires that the following circuithave a large input resistance. Hence the Darlington pair Tr,and Trz is employed.

When vbs goes negative, Trl and with it Tr,, conducts.Transistor TrB then drives Tr, which is a higher power device

Alarm Cir

capable of drawing a relay coil to produce thsignal. At the same time when Trs conducts, the TrS goes negative and hence via positive feedbackthe base ofTra remains negative, even if Vba is zero. Hence, a latching action is obtained, whichwarning signal on. The warning signal will only bif the power supply is removed.

Capacitors C1 and Ca are required to prevenpulses from triggering the alarm, in the case ofprevent switching transients from triggering the athe alarm is being reset, in the case of Ct.

Component valuesR1,R,: 15OkQR,: 2OOkBR,: 25OkQ variableR,: 27kQR,: 47052Ci,C!,:0.33/tF

Tr,, Trz: BC126Tr,: BFR41 or BFDiodes 0A81V,: 18Vvs:9v

Brake light monitor (circuit over)Both of the identical counter-wound coils are wthe reed relay. Hence the relay switch will only ca dashboard warning, if either of the brake lightswith an open circuit or short circuit.


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Water level alarmThis circuit is designed to produce a note from the loud-speaker when the sensor input terminals are shorted. As suchit can be used for many applications apart from suggestedwater level/rain alarm. When the input terminals are shortedbase drive to Tr, via R1 is obtained, and the supply voltageis switched to the unijunction relaxation oscillator com-prising Tr,, R,, R, and C (card 4, series 3). A train of pulses ofperiod mainly determined by the product R&is then presentedto the base ofTr,, thereby producing pulses of current throughthe loudspeaker. The loudspeaker alarm note can be alteredby altering the product R&T. Considerable effective outputcan be obtained by selecting the note to correspond to theresonant frequency of speaker. In practice the alarm willsound for any resistance between zero and five megohms.

The quiescent current of the unit is of the order ofnanoampsso that battery life is many months even if the unit is switchedoff. Provision to test the battery condition is made by switchposition 2 which should cause Trl to switch on the oscillatorprovided the battery is in good condition.

For water level sensing two conducting rods spaced aninch, or less, apart and positioned at the required level is allthat is required.

Component valuesR,: lOOks2R,: 3.3kQR,: 27052C: 0.5,uFTrl, Tr,: 2N2926 (G) Wit&po,i,l0,Tr,: 2N2646 1 -Us.LS: 8-Q loudspeaker

P - tslt3 -ottSupply voltage: 9V f Ra(

For a rain alarm two rods separated by sompaper will suffice. When the blotting paper becontact between the rods is made, the alarm sounwashing is saved once more (provided the missusshopping).

Component changesResistor R1 may be any value up to 5M.Q provshorting of the sensor input terminals is obtainablThe R,C product is dictated by the pitch of the noResistor Rs should be much less than R, e.g. R&OFurther readingAndrews, J. Security Alarm, Practical Electronics,1Moorshead, H. Rain & Water Level Alarm,

Electronics, 1971, p.820.Morum, S. W. F., Brake Light Monitor,Practical 1973, p.588.


Wireless World Circard C


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Wireless World Circard Series 13: Alarm CircuElectromechanical alarmsElectromechanical transducers are obtainable in a widevariety of types: they may be d.c. or a.c., resistive, reluctiveor capacitive, contacting or non-contacting, analogue ordigital, linear or angular, etc. Insofar as most alarm systemsuse a comparator (cross ref. 1) to compare the signal with areference and as d.c. signals are easily compared we shallassume here that any a.c. systems are followed by signalconditioning equipment which includes a rectifier (cross ref. 2)of some sort so that the effective output is d.c.

Displacement alarmOt

in tinCircuit shows a reluctive displacement transducer, of thedifferential transformer type, followed by a demodulator toprovide the d.c. output shown in graph. The core, which isshown in its zero output position, is attached to the memberwhose displacement is required. The core is generally made

from high permeability ferromagnetic material so that fluxlinkages with and hence the e.m.fs of the secondary coils arehighly dependent on the position of the core relative to thecoils. Reluctive transducers generally have a displacementspan of between 0.01 and 120in, in rectilinear form, and

between 0.05 and 90 in angular form. As the induare proportional to frequency, very sensitive systemade at high frequencies.

Capacitive transducers are used in situations wsmall displacements have to be measured ancontacting measurement has to be performed. Phdigital measurements (again non-contacting) are high accuracy is required, although fairly low cocan be constructed if accuracy is not essential.

Velocity alarmLinear velocity transducers are most commonly uvibrations field where the displacement of the memvelocity is required is small. Essentially, they consmoving in a permanent magnetic field, the coil eproportional to the speed. As a large proportion oproducing systems are driven by motors one canobtain information on linear speed from a knoangular speed. This can be obtained by various tyor d.c. tachometers, but with the increasing useinstrumentation, toothed rotor, photoelectric asystems are becoming increasingly common. Diagbasis of operation of the toothed rotor tachomet

corresponding output when the rotor is rotated by the shaft simply provides damping whilst the mass is mo


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corresponding output when the rotor is rotated by the shaftof a motor. The output waveform is obtained because of thechanging flux pattern caused by the changing magnetic circuit.If the output signal is fed to a zero crossing comparator(cross refs. 1, 3) or to a Schmitt trigger (cross ref. 1) one willthen obtain a train of pulses, each pulse representing thepassage of a rotor tooth past the permanent magnet. Obviouslythe pulse frequency is proportional to the shaft speed. If thetrain of pulses is then fed to a frequency-to-voltage convertera direct voltage proportional to shaft speed is obtained andthis can be fed to a comparator to give an alarm if it exceedsa predetermined level. Because the number of teeth on thetoothed rotor can easily be varied, the range of speedsmeasurable by this technique is extremely large. Further,the rotor can easily be constructed in any workshop, no greatprecision being required for many applications. Both headson a coupling between two shafts often suffice as the toothedrotor.

Acceleration alarmAcceleration transducers all have one feature in common vizthe seismic mass, M. The basic acceleration transducer isshown below. The case of the system is attached to the

H. N. Norton, Handbook of Transducers for Measuring Systems, Prentice-Hall.Considine. Encvclonedia of Instrumentation anMcGraw&il l . -_

body whose acceleration is required. Due to a constantacceleration the seismic mass exerts a force MUwhich in the Cross referencessteady state will stretch or compress the spring by an amount x Series 2, Comparators and Schmitts.where Ma = Kx, K being the spring constant. The dashpot Series 4, A.C. Measurements. Series 13, card 4.

0 1974 JPC Business Press Ltd.

simply provides damping whilst the mass is moknow M and K then a measure of x gives a signal pto the acceleration. This can be done by any ditransducer of suitable dimensions and sensitivity. however, the spring arrangement is a leaf spring awith strain gauges attached. The spring deflectionto changes in resistance in the strain gauges whnected in a Wheatstone bridge circuit gives a volttional to the deflection and, hence, to the acceleratWheatstone bridge can be supplied from a d.c. sis no need for rectifiers before feeding to a cStrain gauge bridges usable up to 75OHz have be

For higher frequencies piezoelectric crystals spring. The crystal produces a charge or voltagterminals when subjected to the stress of the seunder acceleration. However, the output impedacrystal is large and amplifiers with an input imexcess of5OOMO typically have to be. used. Furthcable between the crystal and the amplifier requilow capacitance and to be free from friction inducethe other hand very large accelerations (>1OOg) csured and they can be used over a large temper(570C for a lead metaniobate crystal).

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Alarm Circuits-circards 13